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In vitro primary hepatocyte systems typically elicit drug induction and toxicity responses at concentrations much higher than corresponding in vivo or clinical plasma Cmax levels, contributing to poor in vitro-in vivo correlations. This may be partly due to the absence of physiological parameters that maintain metabolic phenotype in vivo. We hypothesized that restoring hemodynamics and media transport would improve hepatocyte architecture and metabolic function in vitro compared with nonflow cultures. Rat hepatocytes were cultured for 2 wk either in nonflow collagen gel sandwiches with 48-h media changes or under controlled hemodynamics mimicking sinusoidal circulation within a perfused Transwell device. Phenotypic, functional, and metabolic parameters were assessed at multiple times. Hepatocytes in the devices exhibited polarized morphology, retention of differentiation markers [E-cadherin and hepatocyte nuclear factor-4α (HNF-4α)], the canalicular transporter [multidrug-resistant protein-2 (Mrp-2)], and significantly higher levels of liver function compared with nonflow cultures over 2 wk (albumin ∼4-fold and urea ∼5-fold). Gene expression of cytochrome P450 (CYP) enzymes was significantly higher (fold increase over nonflow: CYP1A1: 53.5 ± 10.3; CYP1A2: 64.0 ± 15.1; CYP2B1: 15.2 ± 2.9; CYP2B2: 2.7 ± 0.8; CYP3A2: 4.0 ± 1.4) and translated to significantly higher basal enzyme activity (device vs. nonflow: CYP1A: 6.26 ± 2.41 vs. 0.42 ± 0.015; CYP1B: 3.47 ± 1.66 vs. 0.4 ± 0.09; CYP3A: 11.65 ± 4.70 vs. 2.43 ± 0.56) while retaining inducibility by 3-methylcholanthrene and dexamethasone (fold increase over DMSO: CYP1A = 27.33 and CYP3A = 4.94). These responses were observed at concentrations closer to plasma levels documented in vivo in rats. The retention of in vivo-like hepatocyte phenotype and metabolic function coupled with drug response at more physiological concentrations emphasizes the importance of restoring in vivo physiological transport parameters in vitro.

Thrombus formation in the left atrial appendage (LAA) of the heart is a source of stroke risk in patients with atrial fibrillation (Afib). Strategies to prevent thromboembolism from the LAA via restoration of normal rhythm or prevention of clot formation are limited by lack of adequate human surrogate systems that recapitulate the biology ex vivo. Endothelial cell phenotype and function in blood vessels and chambers of the heart are regulated in part by regional hemodynamics. Blood flow shear forces in the LAA are substantially altered during Afib. Thus, we hypothesized that Afib-induced alterations in hemodynamics directly impacts the thrombotic potential of the endothelium in the LAA. To test this, human primary aortic endothelial cell cultures were primed under hemodynamic shear forces simulating the human LAA during normal sinus rhythm for 24 hrs and continued under sinus rhythm or switched to simulate Afib for an additional 24 hrs. Compared to sustained sinus rhythm, Afib hemodynamics suppressed genes (qRTPCR) that define a healthy/quiescent endothelial cell phenotype (e.g. KLF2 and NOS3/eNOS) while inducing the pro-thrombotic gene F3 (tissue factor) and suppressing ADAMSTS13, MMP14 and TIMP2. To determine whether the observed molecular response produced a functional prothrombotic phenotype, hemodynamic primed aortic endothelial cells were exposed to dilute (~27% final concentration) human platelet-free plasma supplemented with Alexa488-labeled human fibrinogen, corn trypsin inhibitor and calcium. Cleavage of fibrinogen to fibrin and concomitant deposition on the endothelium was measured by quantitative confocal microscopy. When exposed to Afib hemodynamics, endothelial cells supported significant fibrin deposition that extended to greater depth (i.e., thicker) on the endothelium compared to sinus rhythm hemodynamics. In conclusion, we show for the first time a direct mechanistic link between changes in LAA hemodynamics associated with development of Afib and functional alterations in the thrombotic potential of the endothelium. This study produced a novel insight enabled by studying potential triggers of thrombosis in a context that more closely mimics the normal and pathological human in vivo environment.